Titration practical method and calculations guide
Chemical changes • Reactions of acids
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Definition and purpose of titration
Titration determines the concentration or amount of a substance by reacting it with a solution of known concentration until the reaction reaches the equivalence point. The equivalence point corresponds to stoichiometric neutralisation of acid and base; the visible endpoint is indicated by a colour change from a suitable indicator. Accurate volume measurement at the endpoint allows calculation of moles and thus concentration or mass of analyte.
Essential apparatus and preparation
Essential apparatus includes a burette, pipette, conical flask, volumetric pipette filler (or bulb), indicator, white tile, and standard solution in a clean, labelled container. All glassware requires rinsing with distilled water; the burette requires rinsing with the titrant and the pipette with the solution to be delivered to remove contaminants and dilute residues. Proper labelling prevents cross-contamination and incorrect reagent use.
Step-by-step practical method
A measured volume of the analyte (usually using a pipette) is placed in a clean conical flask with a few drops of indicator. The burette is filled with standard titrant and initial volume recorded at the bottom of the meniscus to two decimal places in cm3. The titrant is added with swirling until the indicator shows a sustained endpoint colour change; near the endpoint, the titrant is added dropwise to avoid overshooting. The final burette reading records the volume used; the difference produces the titre.
Technique for accurate volume readings
Reading the burette at eye level and recording the bottom of the meniscus removes parallax error; consistent decimal precision improves reproducibility. The conical flask is swirled to mix solutions so reactions proceed uniformly; a white tile under the flask improves endpoint detection by increasing contrast. Multiple concordant titrations (typically three within 0.10 cm3) produce an average titre that reduces random error.
Indicator selection and endpoint vs equivalence point
Indicator choice depends on expected pH change at the equivalence point. Strong acid-strong base titrations show a steep pH change near neutrality, so phenolphthalein or methyl orange gives a clear endpoint. The endpoint is the observed colour change; the equivalence point is the stoichiometric completion. Careful technique minimises the difference between endpoint and equivalence point for accurate results.
Concordant results and repeats
Concordant titres are repeat titre values that lie within a small range (commonly within 0.10 cm3). The first rough titration estimates the endpoint. Subsequent titrations proceed from a closer starting volume and produce precise titres. The mean of concordant titres provides the titre value for calculations and reduces the effect of random variations.
Basic titration calculations (mol/dm3)
Concentration in mol/dm3 equals moles of solute divided by volume in dm3 (concentration = moles / volume). Convert titre in cm3 to dm3 by dividing by 1000. Calculate moles of titrant used from concentration and volume, then use the balanced equation to find moles of analyte. Finally, divide moles of analyte by the pipetted volume (in dm3) to obtain concentration in mol/dm3.
Higher-tier calculations with g/dm3
Mass concentration in g/dm3 represents grams of solute per dm3 of solution. Conversion from g/dm3 to mol/dm3 uses molar mass (Mr): mol/dm3 = (g/dm3) / Mr. For titrations where concentration is given in g/dm3, convert to mol/dm3 before applying stoichiometry. Alternatively, calculate moles from titration then multiply by Mr to find mass of analyte in the original volume, and scale to g/dm3 if required.
Worked example outline
Example: 25.0 cm3 of hydrochloric acid is pipetted into a flask and titrated with 0.100 mol dm−3 sodium hydroxide. Average titre = 23.5 cm3. Convert volumes to dm3: 0.0250 and 0.0235. Moles NaOH = concentration × volume = 0.100 × 0.0235 = 0.00235 mol. Reaction HCl + NaOH → NaCl + H2O gives 1:1 ratio; moles HCl = 0.00235. Concentration HCl = moles / volume = 0.00235 / 0.0250 = 0.0940 mol dm−3. All unit conversions and stoichiometric ratios determine final numeric results.
Common sources of error and limiting factors
Systematic errors include incorrect calibration, contaminated glassware, and misreading the burette. Random errors arise from inconsistent dropwise additions and subjective endpoint detection. Limiting factors for accuracy include purity of reagents, temperature variations affecting volumes, and appropriateness of the indicator. Careful technique and multiple concordant titres reduce these errors.
Key notes
Important points to keep in mind